This invention relates to sub carrier phase shifters for television equipment.
Television signals within a studio complex are normally handled as a signal known as the composite video waveform.
The waveform may be divided into three time regions:
1. The sync pulse which locks the scanning circuits of a television receiver.
2. The colour burst which locks a sub carrier regenerator oscillator to decode the chrominance information.
3. The active picture time during which picture information both chrominance and luminance are transmitted.
Artistic and creative requirements demand that two or more composite signals may be mixed and processed with no apparent time disturbance on the summed output.
During the mixing of two signals the sync pulse and colour burst portions of the video waveform must remain undisturbed which means that the frequency and timing of the two mixed sync pulses must be identical, and that the frequency and phase of the two mixed colour burst must also be equal. If this were not so, a picture shift and chrominance disturbance would be observed during the mixing process. Identical frequencies of the synchronizing components is usually ensured by feeding all picture origination equipment with synchronizing pulses and reference sub carrier generated from a central synchronizing pulse generator. However, to ensure that the colour bursts are of the same phase at the mixing point regardless of system delays, each piece of picture origination equipment must have incorporated a variable phase shifter which can introduce a phase shift in the reference sub carrier path of preferably but not necessarily a 0.degree.-360.degree. shift. It is such a phase shifter that is the subject of the present invention.
Some existing solutions to this requirement are listed below.
A. Cascaded tapped lumped-constant delay lines which may be connected to give the required delay or phase shift. This system does not lend itself to rapid adjustment.
B. A rotating pick up coil which moves within a magnetic field alternating at sub carrier rate. This method provides an easily adjustable control but is an expensive item to produce.
c. Use of high capacitance diodes in which the capacitance varies with voltage (e.g. varicap diodes) within band pass filter networks to vary the delay of such networks. This method is often used since the adjustment can be made by varying a dc level but it can only reasonably provide about 180.degree. of phase adjustment. Sub carrier stability requirements also demand that the varactor diodes be enclosed in a temperature controlled environment.